1. Field of the Invention
This invention relates in general to digital image processing and, more specifically, to a digital image processing system and method for enhancing the edge characteristics of printed or displayed images that are subject to a halftone screening operation. The processed image data may be reproduced on a display or printing system using gray level pixels that vary in size or density depending upon the medium.
2. Description Relative to the Prior Art
Multiple level halftones using gray level pixels are known to improve image quality of pictorial images for lower resolution printers. Different types of halftones are known to improve the stability of the (EP) electrophotographic process (such as full-dot or mixed-dot halftones, see in this regard “Gray Level Printing Method With Embedded Non-Uniformity Correction Method Using a Multi-bit LED Printhead,” IS&T 47 Annual Conference Proceeding, p. 622–625; 1994; and U.S. Pat. No. 5,956,157, the contents of which are incorporated herein by reference thereto) or to improve on the reproduction of detail (such as partial-dot halftone). Image segmentation methods are known to distinguish between the input of local image types so that one can select different halftone types used for different local input image types (such as using a lower screen frequency mixed-mode halftone for pictures and interior of large text in order to improve the EP stability and reduce density contouring, and using a higher screen frequency partial-dot for finer fonts and edges to improve fine line reproduction). Blending of various multiple level halftones in the boundary regions of image types (using fuzzy logic type of approach) is also known to reduce abrupt transition artifacts between segmented image regions, see in this regard U.S. Pat. Nos. 5,200,831; 5,694,224; and 5,956,157. A proper usage of the segmentation methods can produce good EP stability for large area images and produce fine line details, but it does not produce an anti-aliasing effect for near-saturated text and graphics. Anti-aliasing methods such as GRET (Gray Enhanced Anti-aliasing Technology) are known to produce enhanced anti-aliased text/graphics for input saturated text/graphics using multiple level rendering, see in this regard U.S. Pat. Nos. 5,450,531; 5,600,761; and 6,021,256. However, GRET is not very effective to do anti-aliasing for non-saturated text. The inventors have recognized that it would be desirable that the above methods can be merged to produce an anti-aliasing effect for saturated as well as for non-saturated text (even with a multiple color system), to reduce effects from input scanner induced moire (such as input moire caused by high frequency features on images) and still maintain long-term stability of the color electrophotographic process.